<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body {
            border: 1px solid red;
            height: 100%;
            margin: 0;
        }

        div {
            border: 5px solid black;
            height: 100%;
        }

        html {
            height: 100%;
            border: 1px solid blue;
        }
    </style>
</head>
<body>
<!--
    想让一个元素的宽高撑满整个屏幕，就要给整个元素的所有父级都设置宽高100%；

-->
<div></div>
</body>
</html>